Circuit/System Description
WARNING: This page is about a different car, the 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM) has 3 internal 5 V reference busses called 5 V reference 1, 5 V reference 2, and 5 V reference 3. Each reference bus provides 5 V reference circuits for more than one sensor. Therefore, a fault condition on one 5 V reference circuit will affect the other 5 V reference circuits connected to that reference bus. The ECM monitors the voltage on the 5 V reference busses.